The general form of the quasi‐linear equation is derived for plasmas in which there is a finite amplitude pump wave. Electrostatic turbulence is considered and the pump wave is treated in the dipole approximation. Appropriate number, momentum, and energy moment constraints are shown to be computable from the diffusion equation. The derivation of anHtheorem indicates that the quasi‐linear diffusion of parametric instabilities always tends to an accessible asymptotic steady state.